Realtek NIC and xenserver 5.6 FP1

If you cannot connect to the network after upgrade ,see this very  useful post .
It helped me

Edit : In case it disapper from the net ,here is the main issue :

Update 2[solution]
add the following line to /etc/modprobe.conf

options r8169 use_dac=1

reboot XenServer!

How To:

vi /etc/modprobe.conf

press i for “insert”

add options r8169 use_dac=1

press ESC (exit insert mode)

:w (save file)

:q (exit file)

reboot from console

modprobe.conf should look like:

alias eth0 r8169

options r8169 use_dac=1